I've said it before and I'll say it again: swans, geese, and other large waterfowl are not to be trifled with — especially when they're defending a nest. According to the BBC:

Anthony Hensley, 37, worked for a company that provided swans to keep geese away from property.

On Saturday morning, the married father of two set out in a kayak across a pond at a residential complex in Des Plaines, just outside Chicago, where he was tending the birds.

One of the swans charged his boat, capsizing it... Mr Hensley tried to swim to shore but eyewitnesses told the sheriff's investigators the swan appeared to have actively blocked him.

It is thought that Hensley's clothes, shoes, and position in the water rendered him unable to overpower the swan.

According to Chris Perrins, Her Majesty's Swan Warden and retired Oxford ornithologist, such behavior is typical of the intimidating, thirty-pound birds, especially in paired males that have established their springtime nests. Notes Perrins, "They do fairly vigorously defend their little patch this time of year. Once the eggs are hatched he'll stay with the family and defend it."